The Patient Protection and Affordable Care Act (ACA) substantially increased federal funding for federally qualified health centers (FQHCs), providing $11 billion from 2011 to 2015 to expand existing FQHCs and establish new health centers. The goal of this funding was to bolster the supply of primary care in underserved areas to meet some of the anticipated increases in demand for care associated with ACA insurance coverage expansion, particularly among lower-income Americans. Millions of lower-income Americans gained coverage starting in 2014 due to Medicaid expansion and the provision of low-income subsidies for marketplace coverage. To date, however, there is little evidence on whether funding increases for FQHCs are associated with improvements in care or outcomes for the communities they serve. We will examine the effects of changes in federal FQHC funding on three sets of outcomes: 1) outpatient care, including primary and specialty care visits, sources of care (e.g., FQHC vs. non-FQHC), and sentinel preventive care quality measures; 2) downstream clinical events, including emergency department visits (for emergent and non- emergent conditions), hospitalizations (overall and for ambulatory care sensitive conditions), and mortality; and 3) total and component spending (e.g., outpatient, inpatient, pharmacy). We will focus on the experience on Medicaid beneficiaries because they were a central focus of the policy and because we are able to examine comprehensive, beneficiary-level claims data on utilization, diagnoses, and spending over time, and across sites of care for this population. We will use a staggered implementation design with a dose-response model (where dose is determined by the amount of funding) to compare changes in outcomes for beneficiaries living in areas that received larger vs. smaller increases in per capita federal FQHC funding. Because funding levels could be associated with the baseline capacity or quality of the FQHCs, we will use a fixed effects estimation approach to account for unmeasured time-invariant differences across individuals, FQHCs, and local areas. We will also adjust for a range of time-changing covariates to reflect potential changes in individual- (e.g., comorbidities) or area-level demand for care (e.g., insurance coverage mix), as well as area- level provider supply using linked datasets (e.g., American Community Survey). We will examine separately the experiences of: 1) beneficiaries who were continuously eligible for Medicaid (categorically needy), before and after the funding increases (2007-2017); and 2) beneficiaries newly eligible for Medicaid in 2014 (adults with incomes <138% FPL) over the period 2014-2017, during which time there is ongoing variation in FQHC funding. The ACA's Community Health Center Fund was extended in 2016-2017 at $7.2 billion; however, funding after 2017 is uncertain. This study will provide the first information on the effects of changes in FQHC funding during a dynamic period of coverage expansion to provide actionable evidence for policymakers seeking effective and efficient approaches for improving care access and outcomes for vulnerable populations.